Reproducing beam power control for dual-layer recording medium

ABSTRACT

The present invention relates to a method and an apparatus for reading a magneto-optical recording medium comprising a first storage layer and a second storage layer and a read-out layer, wherein an expanded domain leading to a read-out pulse is generated in the read-out layer by copying a mark region from the first or second storage layer to the read-out layer through heating by a radiation power and with the help of an external magnetic field. The radiation power is set to a first value for reading from the first storage layer and to a second value for reading from the second storage layer. A parameter indicating crosstalk between the first and second storage layers is determined during a reading operation, and the radiation power is then controlled on the basis of the determined parameter. Hence, crosstalk between the first and second storage layers can be reduced by keeping the read-out temperature close to the compensation temperature of the other storage layer which is not read. The invention further relates to a recording medium for use in said method and apparatus.

The present invention relates to a multi-layer recording medium,especially a dual-layer recording medium such as a dual-layer MAMMOS(Magnetic AMplifying Magneto-Optical System) disc comprising tworecording or storage layers and one expansion or read-out layer, and toa method and an apparatus for reading such a multi-layer recordingmedium.

In conventional magneto-optical storage systems, the minimum width ofthe recorded marks is determined by the diffraction limit, that is, bythe Numerical Aperture (NA) of the focusing lens and the laserwavelength. A reduction of the width is generally based onshorter-wavelength lasers and higher-NA focusing optics. Duringmagneto-optical recording, the minimum bit length can be reduced tobelow the optical diffraction limit by using Laser Pulsed Magnetic FieldModulation (LP-MFM). In LP-MFM, the bit transitions are determined bythe switching of the field and the temperature gradient induced by theswitching of the laser (or any other suitable radiation source).

In domain expansion techniques, like MAMMOS, a written mark with a sizesmaller than the diffraction limit is copied from a storage layer to aread-out layer upon laser heating and with the help of an externalmagnetic field. Due to the low coercivity of this read-out layer, thecopied mark will expand to fill the optical spot and can subsequently bedetected (during read-out of the recording medium) with a saturatedsignal level that is independent of the mark size. Reversal of theexternal magnetic field collapses the expanded domain. On the otherhand, a space in the storage layer will not be copied and no expansionwill occur. Therefore, no signal will be detected in this case.

To read out the bits or domains in the storage layer, the thermalprofile of the optical spot is used. When the temperature of theread-out layer is above a predetermined threshold value, the magneticdomains are copied from the storage layer to the magneto-staticallycoupled read-out layer. This is because the stray field H_(S) from thestorage layer, which is proportional to the magnetization of this layer,increases as a function of temperature. The magnetization M_(S)increases as a function of temperature for the temperature region justabove a compensation temperature T_(co) where the effectivemagnetization, and thus the stray field of the storage layer, is reducedto zero. This characteristic results from the use of a rareearth-transition metal (RE-TM) alloy which generates two counteractingmagnetizations M_(RE) (rare earth component) and M_(TM) (transitionmetal component) with opposite directions.

The application of an external magnetic field causes the copied domainin the read-out layer to expand so as to give a saturated detectionsignal independent of the size of the original domain. The copyingprocess is non-linear. When the temperature is above the thresholdvalue, magnetic domains are coupled from the storage layer to theread-out layer. For temperatures above the threshold temperature thefollowing condition is satisfied:H _(S) +H _(ext) ≧H _(c)  (1)where H_(S) is the stray field of the storage layer at the read-outlayer, H_(ext) is the externally applied field, and H_(c) is thecoercive field of the read-out layer. The spatial region where thiscopying occurs is called the ‘copy window’. The size w of the copywindow is very critical for accurate read-out. When the condition (1) isnot fulfilled (copy window size w=0), no copying takes place at all. Onthe other hand, an oversized copy window will cause overlap withneighboring bits (marks) and will lead to additional ‘interferencepeaks’. The size of the copy window depends on the exact shape of thetemperature profile (that is, the exact laser power, but also theambient temperature), on the strength of the externally applied magneticfield, and on material parameters that may show short- (or long-) rangevariations.

The laser power used in the read-out process should be high enough toenable copying. On the other hand, a higher laser power also increasesthe overlap of the temperature-induced coercivity profile and the strayfield profile of the bit pattern. The coercivity H_(c) decreases and thestray field increases with increasing temperature. When this overlapbecomes too large, a correct read-out of a space is no longer possibledue to false signals generated by neighboring marks. The differencebetween this maximum and the minimum laser power determines the powermargin, which decreases strongly with decreasing bit length.

In MAMMOS, the synchronization of the external field with the recordeddata is crucial. Accurate clock recovery is possible by using, forexample, data-dependent field switching. Furthermore, the range ofallowed laser powers for correct read-out at high densities is quitesmall. However, this sensitivity to the read-out laser power can also beexploited to achieve an accurate power control loop, that is, dynamiccopy window control, using the read-out signals from the recorded data.This is done by adding a small modulating component (wobbling) to thelaser power, thus inducing timing shifts of the MAMMOS signals. By, forexample, lock-in detection of these shifts, any change in laser power,external field, or ambient temperature can be corrected to keep the copywindow size constant. In this way, an accurate and robust read-out ispossible, allowing much higher densities than with a conventionalsystem. This increase/decrease (wobbling) may be applied with apredefined change pattern, for example a periodic pattern with a smallamplitude. This wobbling causes the copy window to increase or decreasein size synchronously with the wobble frequency. When the copy windowincreases in size, the next transition will appear somewhat earlier thanexpected. On the other hand, when the copy window decreases in size, thenext transition will be delayed slightly. This is indicated by the phaseerror amplitude. This phase error amplitude is a direct measure for theread-out parameter due to a non-linear square-root-like dependence ofthe copy window size on the read-out parameter. To obtain an absoluteerror signal that can be used as an input for the copy window controlloop, the control method requires a suitable reference set-point, whichcorresponds to the optimum read-out parameters such as, for example, theexternal field and/or the laser power.

A major step in capacity has been achieved by using a dual-layer disk.In conventional magneto-optical (MO) systems, different kinds ofdual-layer approaches are known. In most cases, two storage layers areclosely spaced (or even directly connected, that is, exchange coupled)within the focus depth of the objective lens. Read-out of the differentlayers is based on a difference in Kerr rotation and ellipticity. Forexample, the interference layers are adjusted such that a first layeronly gives Kerr rotation, while a second layer only gives Kerrellipticity. Sometimes, different wavelengths are used to improve thiseffect. Another way to read both layers is a kind of multi-levelapproach: depending on the data in the different layers, four differentsignal levels (for example, Kerr rotation) are detected (++, +, −, −−).However, the signal-to-noise-ratio for the medium levels (+, −) islower.

Several options are possible for recording in the different layers. Themagnetic properties may be adjusted such that a first layer has a higherCurie temperature (Tc) than a second layer. In this way, the low-Tclayer can be written at a lower laser power without affecting thehigh-Tc layer. Both layers are affected at a high laser power.Alternatively to or in combination with the above methods, differencesin field sensitivity are used. Here the sign and amplitude of theapplied magnetic field determine the switching of both layers. Forexample, a first layer always follows the sign of the field, whereas asecond layer opposes the field when it is below a certain amplitude andfollows the field when the amplitude is large enough. In this way, bothlayers are written in a single pass. To achieve this behavior, thesecond layer is exchange-coupled to another magnetic layer, for examplea PtCo multilayer or the first storage layer.

Although dual-layer MO is certainly possible, an extension to dual-layerMAMMOS is far from trivial. In the MAMMOS process, a storage layer and aread-out layer are required. Together these layers are at least 30-70 nmthick, which makes the transmission of signals from a read-out layerbelow this set of layers much too low for accurate detection.

Documents WO99/39341 and JP2002-298465 disclose dual-layer MAMMOS discsfor reproducing multi-value signals generated by a combination of strayfields of first and second storage layers in a common read-out layer.Both storage layers are independently read in succession by means of alaser power adapted to heat the non-read storage layer to itscompensation temperature so as to ensure that only the mark of the readstorage layer is copied to the read-out layer. Separate read-out of thedifferent storage layers is thus possible by choosing the correspondingread-out laser power. This laser power should be such that thetemperature of the layer that is not being read is brought close to itscompensation temperature, thus eliminating any stray field influence onthe read-out process.

As was noted above, the laser power and the applied external fieldshould be very carefully balanced by copy window control procedures toenable the highest storage densities in the read-out process of a singlelayer disk. Despite the required tight control (typically around 1% inlaser power), there is quite some room to balance laser power againstexternal field: if the field is somewhat too low, a higher laser powercan still give a correct read-out, and vice versa. This is different,however, in the dual-layer case, because now the storage layer mustreach a predetermined absolute temperature, although it is within moretolerant limits of about ±10° C.

Ideally, every disk and every drive would have perfectly matchedproperties, so that the read power levels in the drive would correspondto the compensation temperatures of the different storage layers. Thisis not the case, however, for several reasons. Apart from contaminationof the drive optics (dust) and, for example, degradation of the laser,the optical (reflectivity, absorption), thermal (conductivity, heatcapacity) and magnetic (T_(co) changes by up to 80 K/at % compositionchange) properties may change from disk to disk and over the radius of asingle disc (non-uniformity in thickness and/or composition). Propercalibration of the read-out parameters corrects for the differencesbetween drives, disks, and disk radii, and allows wider fabricationtolerances. Active copy window control is essential, however, as it isin read-out of single storage layer MAMMOS disks, in realizing a robustread-out at the highest densities. For dual storage layer MAMMOS, laserpower and external field cannot be exchanged freely as in single layerMAMMOS. This is because the read-out temperature has to be kept quiteclose to the compensation temperature of the storage layer that is notbeing read, in order to prevent ‘crosstalk’ from this layer.

It is an object of the present invention to provide a recording mediumand a reading method and apparatus by means of which a proper read-outcan be achieved for dual-layer storage media

This object is achieved by providing a reading apparatus as claimed inclaim 1, by providing a reading method as claimed in claim 17, and byproviding a recording medium as claimed in claim 19.

Accordingly, crosstalk between the first and second storage layers canbe reduced by keeping the read-out temperature close to the compensationtemperature of the other storage layer which is not read.

The determination of the parameter may be based on a detectedcorrelation between a first predetermined data pattern written in thefirst storage layer and a second predetermined data pattern written inthe second storage layer on top of the first predetermined data pattern.Thereby, an initial calibration can be provided for compensation ofdrive-to-drive, disk-to-disk, and radial variations. Starting from apre-set value of the read-out laser power for the first layer, the laserpower is adjusted for the second layer by optimizing read-out of theknown data pattern in that layer. Next, this process can be repeated forthe other storage layer.

As an alternative, the parameter may be based on a detected error in theread-out signal of the first predetermined data pattern written in thefirst storage layer, the error being caused by the second predetermineddata pattern written in the second storage layer. The number ofdeterminations may be determined in response to an information writtenon the recording medium and specifying a characteristic of the recordingmedium. Thus, the manufacturer of the recording medium may provide onthe medium an indication of uniformity to allow for a reduction in thenumber of calibrations to be performed.

Furthermore, the determination may be skipped in response to prior useinformation written on the recording medium, the radiation power thenbeing based on at least one read-out parameter stored on the recordingmedium. The start-up time can thus be reduced in cases where therecording medium has been recently used in this drive. The readingapparatus may be adapted to suppress this skipping operation if aread-out error rate exceeds a predetermined threshold value.

The prior use information may comprise at least one recording mediumidentification stored in the reading apparatus or at least one recordingapparatus identification stored on the recording medium. In particular,the prior use information may be stored together with a correspondingtime and/or date information.

The determination and control means may be adapted to perform theparameter determination and power control at different radii of therecording medium.

As an alternative, the reading apparatus may be adapted to store atleast one read-out parameter or a number of variables for an algorithmdescribing the at least one read-out parameter, as a function of theradius of the recording medium.

The external magnetic field may be controlled on the basis of adifference between the numbers of detected and expected read-out pulses.An independent copy window control is made possible thereby. An increaseor decrease in the number of read-out pulses, which does not correlatewith one of the data patterns, gives independent information on the copywindow size and thus the required correction of the external magneticfield.

It can thus be assured that the first storage layer is readindependently of the second storage layer. The first value of theradiation power is determined by the compensation temperature of thesecond storage layer and the second value of the radiation power isdetermined by the compensation temperature of the first storage layer.

Other advantageous further developments are defined in the dependentclaims.

In the following, the present invention will be described on the basisof preferred embodiments with reference to the accompanying drawings, inwhich:

FIG. 1 is a diagram of a magneto-optical disc player according to apreferred embodiment,

FIG. 2 shows a schematic layer structure of a dual storage layer MAMMOSdisc according to a first embodiment,

FIG. 3 shows a schematic layer structure of a dual storage layer MAMMOSdisc according to a second embodiment,

FIG. 4 shows diagrams indicating temperature dependencies between aread-out layer coercivity and storage layer magnetizations for a firstread-out type,

FIG. 5 shows diagrams indicating temperature dependencies between aread-out layer coercivity and storage layer magnetizations for a secondread-out type,

FIG. 6 shows a diagram indicating the stray field amplitude in theread-out layer as a function of the distance between the storage andread-out layers for different storage layer thicknesses,

FIG. 7 shows a flow diagram of an initial power setting procedureaccording to an embodiment,

FIG. 8 shows an example of a set of data patterns in the first andsecond storage layers, and

FIG. 9 shows a diagram indicating the stray field distribution in theread-out layer as a function of the track direction for the datapatterns of FIG. 8.

FIG. 1 schematically shows the construction of a MAMMOS disc playeraccording to a preferred embodiment. The disc player comprises anoptical pick-up unit 30 having a laser light radiating section forirradiation of a dual storage layer magneto-optical recording medium orrecord carrier 10, such as a dual storage layer MAMMOS disc, with lightthat has been converted, during recording, into pulses with a periodsynchronized with code data. The disc player further comprises amagnetic field applying section comprising a magnetic head 12 whichapplies a magnetic field in a controlled manner during recording andplayback on the magneto-optical disc 10. In the optical pick-up unit 30,a laser is connected to a laser driving circuit that receives recordingand read-out pulses from a recording/read-out pulse adjusting unit 32 soas to control the pulse amplitude and timing of the laser of the opticalpick-up unit 30 during a recording and read-out operation. Therecording/read-out pulse adjusting circuit 32 receives a clock signalfrom a clock generator 26 that comprises a PLL (Phase Locked Loop)circuit.

It is noted that, for reasons of simplicity, the magnetic head 12 andthe optical pickup unit 30 are shown on opposite sides of the disc 10 inFIG. 1. However, they should preferably be arranged on the same side ofthe disc 10.

The magnetic head 12 is connected to a head driver unit 14 and receivescode-converted data via a phase adjusting circuit 18 from a modulator 24during recording. The modulator 24 converts input recording data DI intoa prescribed code.

During playback, the head driver 14 receives a timing signal via aplayback adjusting circuit 20 from a timing circuit 34, wherein theplayback adjusting circuit 20 generates a synchronization signal foradjusting the timing and amplitude of pulses applied to the magnetichead 12. The timing circuit 34 derives its timing signal from the dataread-out operation. Thus, a data dependent field switching can beachieved. A recording/playback switch 16 is provided for switching orselecting the respective signal to be supplied to the head driver 14during recording and during playback.

Furthermore, the optical pick-up unit 30 comprises a detector fordetecting laser light reflected from the disc 10 and for generating acorresponding reading signal applied to a decoder 28 that is arranged todecode the reading signal so as to generate output data DO. Furthermore,the reading signal generated by the optical pick-up unit 30 is suppliedto a clock generator 26 in which a clock signal obtained from embossedclock marks of the disc 10 is extracted or recovered, and which suppliesthe clock signal for synchronization purposes to the recording pulseadjusting circuit 32 and to the modulator 24. In particular, a datachannel clock may be generated in the PLL circuit of the clock generator26. It is noted that the clock signal obtained from the clock generator26 may be supplied to the playback adjusting circuit 20 as well so as toprovide a reference or fallback synchronization which may support thedata-dependent switching or synchronization controlled by the timingcircuit 34.

In the case of data recording, the laser of the optical pick-up unit 30is modulated with a fixed frequency corresponding to the period of thedata channel clock, and the data recording area or spot of the rotatingdisc 10 is locally heated at equal distances. Additionally, the datachannel clock output by the clock generator 26 controls the modulator 24to generate a data signal with the standard clock period. The recordingdata are modulated and code-converted by the modulator 24 to obtain abinary run length information corresponding to the information of therecording data.

In FIG. 1, the timing circuit 34 is provided for supplying adata-dependent timing signal to the playback adjusting circuit 20. Thedata-dependent switching of the external magnetic field mayalternatively be achieved by supplying the timing signal to the headdriver 14 so as to adjust the timing or phase of the external magneticfield. The timing information is obtained from the (user) data on thedisc 10. To achieve this, the playback adjusting circuit 20 or the headdriver 14 is adapted to provide an external magnetic field which isnormally in the expansion direction. When a rising signal edge of aMAMMOS peak is observed by the timing circuit 34 at an input lineconnected to the output of the optical pickup unit 30, the timing signalis supplied to the playback adjusting circuit 20 such that the headdriver 14 is controlled to reverse the magnetic field after a short timeso as to collapse the expanded domain in the read-out layer, and shortlyafter that to reset the magnetic field to the expansion direction. Thetotal time between the peak detection and the field reset is set by thetiming circuit 34 to correspond to the sum of the maximum allowed copywindow and one channel bit length on the disc 10 (times the linear discvelocity).

Furthermore, a dynamic copy window control function is provided byapplying a modulation, e.g. wobble or change pattern, to the head driver14 and continuously measuring the size w of the copy window, usinginformation from the detected data signal in the read mode. If thewobble frequency lies above the bandwidth of the clock recovery PLLcircuit of the clock generator 26, the phase error of this PLL circuitcan be used to detect the small deviation or phase error with respect tothe expected transition position.

The frequency deviation of the introduced wobble or change patternshould have a zero average value. However, the amplitude Δφ of the phaseerror obtained here cannot be used yet as an absolute error signal forlaser power control as only the absolute scale is known, but noreference (zero or offset) is present. That is, only changes in the sizeof the copy window can be measured. To circumvent this problem, thederivative of the copy window size w a function of temperature can bemeasured to obtain control information for controlling the size w of thecopy window. Due to the fact that the derivative or amount of change ofthe copy window size w directly leads to the phase amplitude Δφ, theamplitude Δφ of the detected phase error corresponds to the derivativeand can thus be used for copy window control. The deviation from apredetermined setpoint can then be used as a control signal PE forcontrolling the strength of the external magnetic field at the headdriver 14.

Any changes in the size of the copy window due to changes in parameters,such as coil-disc distance, ambient temperature, etc., are counteractedby the controlled external magnetic field.

In the disc player shown in FIG. 1, a read-out control circuit 290 isprovided, which is adapted to determine or adjust the laser power of theoptical pickup unit 30. According to a preferred embodiment, the laserpower is controlled by the read-out control circuit 290 independently ofthe copy window control at the clock generator 26, which may be based onthe external magnetic field, laser power, or any other suitable read-outparameter. In particular, the read-out control circuit 290 determines aparameter that is a suitable and reliable indicator for crosstalkbetween the first and second storage layers of the MAMMOS disc 10.

FIG. 2 shows a layer structure of a dual storage layer MAMMOS discaccording to a first embodiment. The solution proposed here is to useonly one read-out layer 106 to reproduce the information in thedifferent storage layers 110, 114 arranged one on top of the other. Theread-out layer 106 is arranged on top of the two storage layers 110, 114in the direction of the laser incident side. Recording of these storagelayers 110, 114 is possible by any of the methods described in the priorart. The main difficulty is to fulfill the MAMMOS read-out requirementson the balance of coercivity, stray field (from storage layer at theread-out layer,) and applied external field, i.e. for both storagelayers 110, 114. For MAMMOS reproduction of a mark bit, the magneticproperties of the storage and read-out layers 106, 110, 114, and thelaser power for read-out are chosen such that the sum of the stray fieldgenerated by the mark and the applied external field is just greaterthan the coercivity of the read-out layer, that is, H_(S)+H_(ext)>H_(c).As both storage layers 110, 114 produce a stray field, the equation canbe modified as follows:H _(S1) +H _(S2) +H _(ext) >H _(c,)  (2)wherein H_(S1) and H_(S2) designate respective stray field strengths ofthe storage layers 110, 114.

To allow separate read-out of both storage layers, one layer being readwithout being influenced by the other layer, the layer structure shownin FIG. 2 is proposed according to the first embodiment of the dualstorage layer MAMMOS disc. Starting from the laser incidence side, thegeneric layer stack comprises an optional first cover or substrate 102,a first dielectric layer 104 made of, for example, SiN, SiO₂, and theread-out layer 106, preferably made of GdFeCo or GdFe with a thicknessof 10-30 nm, preferably 20 nm. Furthermore, a non-magnetic spacer layer108 with a thickness of 1-15 nm, preferably 5 nm, and made of, forexample, SiN or Al is provided between the read-out layer 106 and thefirst storage layer 110. The first storage layer 110 has a thickness ofpreferably 8-35 nm and is preferably made of TbFeCo, possibly with theadditions of rare earth, transition or other metals, non-metals such asSi, etc. An optional intermediate layer 112 is arranged between thefirst storage layer 110 and the second storage layer 114. Theintermediate layer 112 may be a non-magnetic dielectric or metal spacerlayer with a thickness of 1-15 nm, preferably 5 nm, or a Ru exchangecoupling layer with a thickness of 0.1-5 nm. As a further alternative,no intermediate layer 112 may be used at all, such that a directexchange coupling is provided between the first and second storagelayers 110, 114.

The second storage layer 114 may have a thickness of preferably 10-100nm and may be preferably made of TbFeCo, possibly with additions asdescribed above in connection with the first storage layer 110.Additionally, an optional exchange bias layer 116, for example amultilayer of PtCo or PdCo, amorphous RE-TM material, etc. may beprovided, followed by a second dielectric layer 118 made of SiN or SiO₂and including an optional heat sink. Finally, an optional secondsubstrate or cover 120 is provided. The first and second storage layers110 and 114 should have at least the following magnetic properties:

-   -   ferrimagnetic with different compensation temperatures T_(co1)        and T_(co2), both below the respective Curie temperatures T_(c1)        and T_(c2),    -   internal drive temperature T_(ambient)(<˜70°        C.)<T_(co1)≠T_(co2)<min(T_(c1), T_(c2)), and    -   read-out temperatures T_(read-out1)=T_(co2) and        T_(read-out2)=T_(co1), while the differences should be smaller        than approximately 10° C. to avoid interlayer crosstalk. Larger        differences will limit the possible storage density.

FIG. 3 shows a schematic layer structure of a dual storage layer MAMMOSdisc according to a second embodiment. In this second embodiment, theread-out layer 106 is placed between the first and second storage layers110, 114. In that case, the first storage layer 110 which is closest tothe laser incidence side should preferably be thinner than approximately10 nm and the dielectric layer(s) 104, 112 (optical interference) shouldpreferably be adjusted to maximize the Kerr signal from the read-outlayer 106 while suppressing that from the upper first storage layer 110.

FIGS. 4 and 5 show diagrams indicating temperature dependencies betweena read-out layer coercivity H_(c) and storage layer magnetizations M forrespective first and second embodiments. The magnetization curvesrelating to the first storage layer 110 are indicated by solid lines,and the magnetization curves relating to the second storage layer 114are indicated by dashed lines. M_(1,1) means magnetization of the firststorage layer 110 at the read-out temperature of the first storage layer110, which is equal to the compensation temperature T_(co2) of thesecond storage layer 114. Similarly, M_(2,2) means magnetization of thesecond storage layer 114 at the read-out temperature of the secondstorage layer 114, which is equal to the compensation temperatureT_(co1) of the first storage layer 110.

According to the proposed read-out scheme, read-out of the first storagelayer 110 is achieved by having the read-out control circuit 290 of FIG.1 adjust the laser power to heat the second storage layer 114 to itscompensation temperature T_(co2). Since the effective magnetization Mvanishes at this temperature, the stray field contribution H_(S2) fromthe second storage layer 114 also becomes zero. Thus, only the strayfield contribution H_(S1)generated by the bits in the first storagelayer 110 will trigger the MAMMOS copy and expansion read-out process.The same principle can be used for read-out of the second storage layer114, i.e. the read-out control circuit 290 adjusts or changes the laserpower to heat the first storage layer 110 to its compensationtemperature T_(co1) which will suppress H_(S1) and allow separate orindependent read-out of the data in the second storage layer 114. Thissimple layer selection method does not require any modifications to theoptics of the optical pickup unit 30, i.e. no focus jumps, aberrationcorrection, etc., and only very minor adjustments in the electronics areneeded compared with a single-layer system.

From this read-out method it is clear that the read-out temperatures,and thus both compensation temperatures, should be above the (maximum)ambient temperature. Both compensation temperatures should also be belowthe lowest of the storage layers' Curie temperatures because a read-outtemperature close to (or higher than) the Curie temperature may disturbor erase the data in the respective layer, especially when magneticfields are applied.

The diagrams of FIG. 4 relate to the first embodiment ofexchange-coupled storage layers with different field sensitivities,where the first storage layer 110 is read out at a lower temperatureT_(read-out1)=T_(co2) while the second storage layer 114 is read out ata higher temperature T_(read-out2)=T_(co1). The Curie temperaturesT_(c1) and T_(c2) of the first and second storage layers 110, 114 areequal. The diagrams of FIG. 5 relate to the second embodiment of,separate, de-coupled storage layers with different Curie temperatures,where the first storage layer 110 is read out at a higher temperatureT_(read-out1)=T_(co2) while the second storage layer 114 is read out ata lower temperature T_(read-out2)=T_(co1). Here, the Curie temperatureT_(c1) of the first storage layer 110 is lower than the Curietemperature T_(c2) of the second storage layer 114.

To enable the MAMMOS read-out process, a number of additional conditionsshould be fulfilled by the combination of layer stack and magneticproperties of the read-out and storage layers 106, 110, 114:

-   -   the external magnetic field H_(ext) used during read-out should        be sufficiently strong to drive the domain expansion process.        For reasons of simplicity, it is preferable (but not necessary)        that H_(ext) is the same for both storage layers. Practical        field strengths are between 8 and 16 kA/m, but may be lower or        higher,    -   at each of the read-out temperatures, the coercivity of the        read-out layer 106 (H_(c1) for read-out of the first storage        layer 110, H_(c2) for read-out of the second storage layer 114)        should be greater than the applied external field, i.e.        H_(c1)>H_(ext1) and H_(c2)>H_(ext2), or min(H_(c1);        H_(c2))>H_(ext). If this condition is not fulfilled, the        read-out process will no longer be determined only by the data        in the storage layer, i.e. the read-out layer's magnetization        will ‘follow’ the applied magnetic field instead of the data,    -   the minimum strength of the stray fields generated by the data        in the storage layers 110, 114 which is required in the read-out        layer 106 is determined by the difference H_(c)−H_(ext). Thus,        H_(S1)>H_(c1)−H_(ext1) and H_(S2)>H_(c)2−H_(ext2). These stray        fields depend on the respective magnetizations M_(1,1) and        M_(2,2) of the storage layers 110, 114 at their read-out        temperatures (as explained earlier, M_(1,2) and M_(2,1) should        preferably be close to zero), the respective thicknesses t1 and        t2 of the storage layers 110, 114, and the respective distances        D1 and D2 between the storage layers 110, 114 and the read-out        layer 106.

FIG. 6 shows a diagram indicating the stray field amplitude H_(S) in theread-out layer 106 generated by bits in the storage layer as a functionof the distance between the storage and read-out layers for differentstorage layer thicknesses t ranging from 10 nm (solid line) to 50 nm(dotted line) and for a magnetization M=100 kA/m. As shown in FIG. 2, athicker layer gives a stronger stray field H_(S), but this field rapidlydecreases at a greater distance.

Assuming realistic values for the magnetization M, this means that inthe first embodiment of FIG. 2 the upper, first storage layer 110 shouldpreferably be thinner than the lower, second storage layer 114. Toenable reliable and high-density recording, the storage layers 110, 114should preferably have a thickness between 8 and 100 nm. Thicker layersare possible, but at the cost of some density. Typical values for thefirst storage layer 110 may be between 10 and 35 nm, and for the secondstorage layer 114 between 10 and 100 nm.

All the layers shown in FIGS. 2 and 3 can be sputtered usingconventional equipment. Only a few additional layers (typically 1 to 3)are needed compared with a single-layer MAMMOS disk. Normally, there isno need for a spin-coated or PSA (Pressure Sensitive Adhesive) spacerlayer with tight tolerances and related spherical aberration problems.Moreover, as was noted above, no modifications are required to theoptical system, only minor modifications to electronics, i.e. to theread-out control circuit 290 for switching the laser power for read-outof the different storage layers 110, 114. Assuming an allowed residualstray field due to a non-zero magnetization of 1 kA/m, alloweddeviations were estimated in the order of ±10° C. Compared with theread-out power margin of ˜1%=>1.5° C., this is quite tolerant. Suchallowed deviations also pose no problems to the manufacture of suchdisks.

In the following, examples of stack designs of the above first andsecond embodiments (as shown in FIGS. 4 and 5) are given for λ=405 nmand a numerical aperture NA=0.85:

For the first embodiment (shown in FIG. 4):

45 nm SiN

20 nm GdFeCo

5 nm SiN

20 nm TbFeCo,1

5 nm SiN

50 nm TbFeCo,2

20 nm CoPt multilayer

20 nm SiN

30 nm Al alloy

substrate

This stack design according to the first read-out type leads to thefollowing read-out parameters: T_(co1)=150° C., T_(co2)=130° C.,T_(c1)=T_(c2)=200° C., H_(c1)=35kA/m, M_(1,1)=90kA/m, H_(c2)=25kA/m,M_(2,2)=50 kA/m, and H_(ext)=16kA/m.

For the second embodiment (shown in FIG. 5):

45 nm SiN

20 nm GdFeCo

5 nm SiN

10 nm TbFeCo,1

5 nm SiN

50 nm TbFeCo,2

20 nm SiN

30 nm Al alloy

substrate

This stack design according to the second read-out type leads to thefollowing read-out parameters: T_(co1)=150° C., T_(co2)=130° C.,T_(c1)=200° C., T_(c2)=250° C. , H_(c1) =25 kA/m, M _(1,1) =70 kA/m, H_(c2)=35 kA/m, M_(2,2)=90 kA/m, and H_(ext)=16 kA/m.

Other variations, for example with switched low and high temperatures,are also possible.

It is clear from the above that the read-out laser power levels for bothstorage layers have to be calibrated in some way. A good way to do thisis to record or write a first known or predetermined data pattern in thefirst storage layer 110 and a second known or predetermined data patternin the second storage layer 114, directly below the first pattern in thefirst storage layer 110. Depending on the above two media types, i.e.exchange-coupled layers with different field sensitivities (cf. FIG. 4)or separate, de-coupled layers with different Curie temperatures (cf.FIG. 5) and the related recording methods, this may be done in a singlepass or in two passes.

FIG. 7 is a flow diagram of an initial power setting or calibrationprocedure according to the preferred embodiment. Starting from a pre-setor default value of the read-out laser power for the first storage layer110 (step S201), the first known data pattern is read from the firststorage layer 110 (step S202), and the laser power is adjusted by theread-out control circuit 290 to minimize the crosstalk effects of thedata pattern in the second storage layer 114 (step S203). In particular,the read-out signal is compared with the first known data patternprovided in the first storage layer 110. The optimum laser powercorresponds to the best match, for example characterized by across-correlation method or by a minimum in Bit Error Rate, and isstored. Then this process is repeated in steps S204 to S206 for thesecond storage layer 114 and the second known data pattern of the secondstorage layer 114.

It may be advantageous to evaluate both known data patternssimultaneously while adjusting the read-out laser power. The best matchto the first known data pattern then leads to the optimized read-outpower for the first storage layer 110, and the best match to the secondknown pattern leads to the optimized read-out power for the secondstorage layer 114. Since it is known which power is higher, the searchdirection, i.e. higher or lower power, and even a good guess of theother laser power can be made as soon as one of the data patterns hasbeen matched. The advantage of this approach is that it is faster andmay allow shorter and/or less calibration areas, thus reducing thecapacity overhead.

FIG. 8 shows an example of a useful set of data patterns in the firstand second storage layers, where “+1” and “−1” indicate themagnetization directions of individual bits. The correct output for thefirst storage layer 110 would be a single alternating bit pattern.However, if the magnetization of the second storage layer 114 is toostrong, additional MAMMOS signal peaks will appear in the parts wherethe second storage layer 114 has “+1” data, due to the fact that thetotal stray field becomes too strong. On the other hand, some peaks willbe suppressed in those parts where the second storage layer 114 has “−1”data, due to the fact that the total stray field is not sufficient totrigger a read-out process.

FIG. 9 shows a diagram indicating the stray field distribution in theread-out layer as a function of the track direction for the datapatterns of FIG. 8 in the case of a read-out operation of the firststorage layer 110. The horizontal dotted line indicates the optimumstray field value for the MAMMOS read-out. The sinusoidal solid line p1corresponds to the stray field caused by the first data pattern in thefirst storage layer, while the rectangular dashed line P2 corresponds tothe stray field caused by the second data pattern of the second storagelayer 114 at a non-zero magnetization (M>0). It is clear that anyinfluence or crosstalk from the second storage layer 114 due to thenon-zero magnetization M at read-out temperature will cause a change inthe total stray field indicated by the dotted sinusoidal line p1+p2. Ifthis influence is in excess of a certain value (e.g. 1 kA/m), the firstdata pattern is no longer correctly reproduced.

To relax the manufacturing tolerance of the discs, for example matchingof magnetic properties to stack thickness of various layers: tuningH_(c) vs. T with H_(S1), H_(S2) and H_(ext), it is advantageous also toadjust the external field amplitude H_(ext) during the power setting orcalibration procedure. Changing the laser power will not only influencethe contribution from the other storage layer, but will also change thesize of the copy window, and with that the read-out resolution. Only aperfectly matched disk 10 will reach the optimum copy window at the samelaser power at which it reaches the correct read-out temperature, i.e.the compensation temperature of the other storage layer.

To achieve this relaxed tolerance, the power setting or calibrationprocedure must allow a (largely) independent control of the copy windowand the suppression of the other storage layer. For the latter, theprocedure remains very similar to what has been described above. Foreach storage layer, the laser power is tuned so that the first knowndata pattern is correctly reproduced and no effect of the second datapattern is detected. In the example of FIG. 9, any effect of the seconddata pattern will be visible as a signal with the same period as thesecond data pattern, i.e. 2×4 bits and, alternatingly, too many and toofew peaks.

For example, auto-correlation techniques may be used to detect this. Toachieve an independent copy window control, tuning of the external fieldshould preferably be based on the number of detected MAMMOS peaks. Forexample, additional peaks will appear if the copy window is too large;some or all peaks will be missing if the copy window is too small. Thisincrease or decrease in the number of peaks, however, does not correlatewith the second data pattern, provided that the second data pattern is,for example, not the same as the first data pattern, which shouldobviously be avoided, and therefore gives independent information on thecopy window size and thus the required correction of the external fieldH_(ext). If too many peaks are detected, H_(ext) should be reduced, andif too few peaks are detected, H_(ext) should be increased. In FIG. 1,this control can be performed by means of the control signal PE.

Examples of possible system implementations will be described below.According to a first option, one or more different radii can becalibrated upon insertion of the disk 10. This may be necessary forcompensating for radial non-uniformity. Read-out power level andpossibly H_(ext) parameters are stored in the drive and/or on disk 10.These parameters may be a set of amplitudes with radial positions, or anumber of variables for an algorithm describing the laser power andpossibly H_(ext) as a function of the radius, for example start, middleand end value, plus some curvature or order parameter.

According to a second option, the disk manufacturer may providealgorithm variable information on the disk, thus allowing a furtherreduction in the number of calibrations to be performed. For example, ifa disk is highly uniform, one or two calibrations are sufficient for thewhole disk, whereas a non-uniform disk needs many more.

According to a third option, it is checked upon insertion of the disk,whether this disk has been recently used in this drive. If it has, powersetting or calibration is skipped to reduce the start-up time and thestored parameters (if available) are used to determine the read-outpower levels and possibly H_(ext). Afterwards, calibration is onlyperformed in case of high error rates. Thus, this option may requirestoring the disk's ID in the drive or storing the drive's ID on thedisk, for example, by reserving space for five drives, overwriting theoldest if necessary, preferably along with some time/date informationand the laser power parameters and possibly H_(ext). This may preventinitial read-out problems, for example if the disk 10 has beenpreviously used in a drive that has updated the disk's power parameterfields with different settings.

It is noted that the present invention may be applied to any readingsystem for domain expansion magneto-optical disc storage systems forreading from two storage layers. Layer stacks and read-out methodssimilar to those proposed above may also be used in systems with, forexample, card-shaped media, non-moving, stationary read-out principlesbased on arrays of optical spots and/or thin-film magnetic sensors (suchas GMR or TMR), or alternative local heating methods such as, forexample, addressable crossed metal wires inside or brought close to themedia.

The read-out control circuit 290 may be implemented by a hardwarecircuit or alternatively by a software-controlled analog or digitalprocessing circuit, or may be incorporated as a new routine in anexisting control program for controlling the disc player. Theembodiments may thus vary within the scope of the attached claims.

1. A reading apparatus for reading from a magneto-optical recordingmedium (10) comprising a first storage layer (S1) and a second storagelayer (S2) and a read-out layer (RO), wherein an expanded domain leadingto a read-out pulse is generated in said read-out layer (RO) by copyinga mark region from said first or said second storage layer to saidread-out layer through heating by a radiation power and with the help ofan external magnetic field, said apparatus comprising: a) setting means(290) for setting said radiation power to a first value for reading fromsaid first storage layer and to a second value for reading from saidsecond storage layer; b) determination means (290) for determining aparameter indicating crosstalk between said first and second storagelayers; and c) control means (290) for controlling said radiation poweron the basis of said determined parameter.
 2. A reading apparatusaccording to claim 1, wherein said determination means (290) is adaptedto determine said parameter from a detected correlation between a firstpredetermined data pattern written in said first storage layer (S1) anda second predetermined data pattern written in said second storage layeron top of said first predetermined data pattern.
 3. A reading apparatusaccording to claim 1, wherein said determination means (290) is adaptedto determine said parameter from a detected error in a read-out signalof a first predetermined data pattern written in said first storagelayer (S1), said error being caused by a second predetermined datapattern written in said second storage layer, said first and secondpredetermined data patterns being written one on top of the other.
 4. Areading apparatus according to claim 1, wherein said control means (290)is arranged to control the number of determinations by saiddetermination means (290) in response to an information written on saidrecording medium and specifying a characteristic of said recordingmedium (10).
 5. A reading apparatus according to claim 4, wherein saidcharacteristic specifies a uniformity of said recording medium.
 6. Areading apparatus according to claim 1, wherein said control means (290)is adapted to skip said determination by said determination means (290)in response to a prior use information, and to control said radiationpower on the basis of at least one read-out parameter stored on saidrecording medium (10).
 7. A reading apparatus according to claim 6,wherein said reading apparatus is adapted to suppress said skippingoperation if a read-out error rate exceeds a predetermined thresholdvalue.
 8. A reading apparatus according to claim 6, wherein said prioruse information comprises at least one recording medium identificationstored in said reading apparatus or at least one recording apparatusidentification stored on said recording medium (10).
 9. A readingapparatus according to claim 8, wherein said prior use information isstored together with a corresponding time and/or date information.
 10. Areading apparatus according to claim 1, wherein said determination andcontrol means (290) are adapted to perform said parameter determinationand power control at different radii of said recording medium.
 11. Areading apparatus according to claim 1, wherein said reading apparatusis adapted to store at least one read-out parameter or a number ofvariables for an algorithm describing the at least one read-outparameter as a function of the radius of said recording medium (10). 12.A reading apparatus according to claim 1, wherein said reading apparatusis adapted to control said external magnetic field on the basis of adifference between the numbers of detected and expected read-out pulses.13. A reading apparatus according to claim 1, wherein said control means(290) is adapted to control said radiation power so as to minimize saidparameter.
 14. A reading apparatus according to claim 1, wherein saidreading apparatus is adapted to read out said first storage layer (S1)independently of said second storage layer (S2).
 15. A reading apparatusaccording to claim 14, wherein said first value of said radiation poweris determined by a compensation temperature of said second storage layer(S2) and said second value of said radiation power is determined by acompensation temperature of said first storage layer (S1).
 16. A readingapparatus according to according to claim 1, wherein said control means(290) is adapted to perform said power control independently of a fieldcontrol of a copy window used for mark copying.
 17. A method of readinga magneto-optical recording medium (10) comprising a first storage layer(S1) and a second storage layer (S2) and a read-out layer (RO), whereinan expanded domain leading to a read-out pulse is generated in saidread-out layer (RO) by copying a mark region from said first or saidsecond storage layer to said read-out layer through heating by aradiation power and with the help of an external magnetic field, saidmethod comprising the steps of: a) setting said radiation power to afirst value for reading from said first storage layer and to a secondvalue for reading from said second storage layer; b) determining aparameter indicating crosstalk between said first and second storagelayers; and c) controlling said radiation power on the basis of saiddetermined parameter.
 18. A method according to claim 17, wherein saidparameter is determined during an initial calibration based on adetected correlation between a first predetermined data pattern writtenin said first storage layer (S1) and a second predetermined data patternwritten in said second storage layer on top of said first predetermineddata pattern.
 19. A recording medium comprising a first storage layer(S1), a second storage layer (S2), and a read-out layer (RO), wherein anexpanded domain leading to a read-out pulse is generated in saidread-out layer by copying a mark region from said first or said secondstorage layer to said read-out layer through heating by a radiationpower and with the help of an external magnetic field, said recordingmedium (10) having written thereon a first predetermined powercalibration data pattern in said first storage layer and a secondpredetermined power calibration data pattern in said second storagelayer, wherein said first and second data patterns are arranged one ontop of the other.
 20. A recording medium according to claim 19, whereinsaid read-out layer (RO) is arranged on top of said first and secondstorage layers (S1, S2).
 21. A recording medium according to claim 19,wherein said read-out layer (RO) is arranged between said first andsecond storage layers (S1, S2).
 22. A recording medium according toclaim 19, wherein said first and second data patterns are mutuallydifferent data patterns.
 23. A recording medium according to claim 19,wherein said first and said second data patterns are both written atseveral locations of said recording medium (10).
 24. A recording mediumaccording to claim 19, wherein said recording medium comprises an areafor storing at least one read-out parameter or a number of variables foran algorithm describing the at least one read-out parameter as afunction of its radius.